1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) based wireless communication system, and more particularly to a method for cancelling Inter-Channel Interferences (ICIs) using a precoding technique.
2. Description of the Related Art
Recently, OFDM has been considered as representative fundamental technology of the next generation mobile communication system for supporting a requisite transfer rate of the next generation mobile communication service. The OFDM system compensates for frequency selective channels using a plurality of simple one-tap equalizers, such that it has simple equalization complexity. The OFDM system enables individual subcarrier bands to overlap with each other using orthogonality characteristics, resulting in high bandwidth efficiency. Therefore, the conventional broadband systems such as wireless LANs and DSLs have adopted the OFDM system as a data transmission scheme. However, in the mobile communication system using the OFDM, a channel variation between a transmission end and a reception end, due to the movement of the user, causes a serious problem.
The next generation mobile communication service uses a high frequency band of a maximum 5 GHz, and has a predetermined speed of 250 km per hour as a maximum moving speed of a user, resulting in the occurrence of a considerably high channel variation. If the OFDM system is used to obviate the aforementioned problem, there arises a channel variation in an OFDM symbol, such that an unexpected ICI occurs because of lost inter-channel orthogonality, resulting in deterioration of system performance.
In other words, an ICI occurs on the condition that a channel variation occurs in a single OFDM symbol interval. Therefore, in the case of using a general one-tap equalizer, there arise structural problems, for example, deterioration of detection performance, and a high Bit Error Rate (BER), because channel estimation is performed assuming that the inter-channel orthogonality is maintained even though it is not maintained in a real condition.
The research method for solving the aforementioned problems is largely classified into two research methods. A first research method uses modulation/demodulation methods to cancel ICI. A second research method is a new architecture equalization scheme, which performs channel modeling according to time-varying characteristics to perform channel estimation and uses a time-varying channel parameter.
A representative ICI cancellation method that is well known in the art is an ICI-self-cancellation modulation/demodulation scheme. In a modulation mode, the ICI-self-cancellation modulation/demodulation scheme transmits signals to be transmitted over even subcarriers, and transmits negative(−) values of the signals transmitted to the even subcarriers to odd subcarriers. In a demodulation mode, the ICI-self-cancellation modulation/demodulation scheme subtracts signals received in the odd subcarriers from the other signals received in the even subcarriers in such a way that it can recover the original signal. The demodulated subcarrier reception signal cancels its own ICI in such a way that a transmission signal passes through a single-path channel using an approximation method. The aforementioned ICI cancellation method uses a DPSK (Differential Phase Shift Keying) scheme, and does not consider time-varying channel estimation.
However, the aforementioned ICI cancellation method uses only even subcarriers to transmit data, and uses odd subcarriers to cancel ICI, resulting in 50% band efficiency. Also, the ICI cancellation method uses the DPSK modulation scheme, resulting in lower frequency selectivity. Because the frequency selectivity makes a double effect to the DPSK performance, the low frequency selectivity significantly deteriorates the system performance.
Further, if the total number of subcarriers increases to lower the frequency selectivity, the length of OFDM symbol interval also increases, such that a high channel variation occurs in a single OFDM symbol interval and an ICI excessively increases to make it impossible to perform ICI cancellation, resulting in system performance deterioration.
A conventional time-varying channel estimation/equalization method proposed an improved method capable of modeling a channel variation in the OFDM symbol, and estimating a time-varying channel parameter using a pilot subcarrier. The conventional time-varying channel estimation/equalization method also disclosed a method for arranging pilot subcarriers to acquire optimum channel estimation performance. Although this channel estimation method can estimate a channel varying with time, it unavoidably deteriorates channel estimation performance due to the increasing ICI when the channel variation increases. In this case, although the number of pilot subcarriers may increase to improve the channel estimation performance, a large number of pilot subcarriers are required to reduce the ICI influence, resulting in serious deterioration of band efficiency.
Provided that the time-varying channel estimation value is given, the OFDM system may use a variety of time-channel equalization methods. Provided that the total number of subcarriers of the OFDM system is set to N, a transmission signal vector is set to X=[X(0), . . . , X(N−1)]T, a reception signal vector is set to Y=[Y(0), . . . , Y(N−1)]T, and an AWGN noise vector having dimensions of N×1 is set to W, the relationship between transmission and reception signals is denoted by Equation 1.Y=HX+W  (1)
The N×N matrix H, which is in the form of a diagonal matrix with the orthogonality between the subcarriers, shows as a matrix of which entries are fully non-zero under the influence of the ICI. Equalization is a process for finding an X-vector from a Y-vector using H-matrix entries acquired from the time-varying channel estimation value.
In a conventional equalization technique, the matrix H is considered with fully non-zero entries so as to use an inverse matrix of H for equalization. Other known equalization technique uses the characteristic in that the diagonal entries and their peripherals of the matrix H have high value relative to remaining ones even when the ICI exists, so as to simplify the inverse matrix calculation complexity by assuming that the diagonal entries and their peripherals are non-zero. However, the first equalization technique requires much calculation complexity in proportional with the number of subcarriers such that it is unpractical in real system. Additionally, the second technique has a problem in that the equalization performance deteriorates due to the difference between the assumption of large ICI and the real ICI environment.
The equalization technique for a time-varying channel has more disadvantages in that it unavoidably increases channel estimation error due to the ICI especially when the mobile terminal moves in high velocity.
As described above, the conventional ICI cancellation modulation/demodulation techniques has drawbacks in that the low frequency selectivity decreases the system performance and the utilization of the half of total subcarriers for ICI cancellation decreases the bandwidth efficiency.
Also, the conventional equalization technique has drawbacks in that the estimation performance considerably decreases in the time varying channel due to the large ICI and the bandwidth efficiency decreases when increasing the number of the pilot subcarriers for enhancing the channel estimation performance.